Little consistency has been manifest among investigators in choosing an appropriate experimental model for maxillofacial bone research. In an effort to develop a protocol for the experimental analysis of maxillofacial nonunions, previous studies using calvarial and mandibular defects as models were reviewed. The creation of nonunions in animals within the calvaria and mandible was size dependent. Defects of a size that will not heal during the lifetime of the animal may be termed critical size defects (CSDs). A rationale was postulated for testing bone repair materials (BRMs) using CSDs in a hierarchy of animal models. This rationale suggests that testing should be initiated in the calvaria of the rat and rabbit, followed by testing in the mandibles of dogs and monkeys. While calvarial CSDs have been established in the rat, rabbit, and dog, further research is necessary to determine the CSD in the calvaria of the monkey, as well as the mandibles of dogs and monkeys.